The MRI apparatus is an imaging diagnostic apparatus which magnetically excites nuclear spins of an object set in a static magnetic field with RF (radio frequency) signals having the Larmor frequency and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
As one of image generating methods in an MRI apparatus, real part imaging is known. The real part imaging is an image generating method for performing imaging processing using not absolute values but real parts of complex signals to be an imaging target. In real part imaging, a technique for selectively acquiring MR signals from blood as negative signals by applying a single or multiple IR (inversion recovery) pulses is devised.
According to this method, intensities of MR signals from blood which flows inside blood vessels can be suppressed. For this reason, real part imaging of blood with application of an IR pulse or IR pulses makes it possible to depict the blood as low signal regions with an improved CNR (contrast to noise ratio). In this case, the background tissues are depicted whitely as high signal regions while blood is depicted blackly.
Such a blood flow image on which blood is depicted blackly is called BB (black blood) image. The BB image can be acquired as a longitudinal relaxation (T1) weighted image (T1W), a transverse relaxation (T2) weighted image (T2W), or a proton density weighted image (PDW).
When a BB image is acquired as a T1W, an IR pulse is applied as a prepulse to a slab larger than an imaging slab to be imaged before MR data for imaging are acquired. Thereby, the longitudinal magnetizations Mz of static tissues and blood in an application region of the IR pulse are inverted. Then, a TI (inversion time) is set so that imaging data are acquired at timing before timing at which the longitudinal magnetization Mz of the blood becomes zero due to the longitudinal relaxation (T1 relaxation). Note that, the TI is a time from an application timing of an IR pulse to that of an RF excitation pulse for acquiring imaging data.
Moreover, an application region of an IR pulse is set to be a region, which is larger than an imaging slab, considering a flow velocity of blood and the T1 relaxation of the blood so that blood flowing from the outside of the imaging slab into the imaging slab becomes a target of the IR pulse application at an acquisition timing of imaging data. That is, blood which flows into an imaging slab at an acquisition timing of imaging data is also an inversion target of the longitudinal magnetization Mz by the application of an IR pulse.
When a readout of MR signals is performed by a gradient echo (GRE) type of sequence, such as a fast field echo (FFE) sequence, under such conditions, MR signals from the background tissues become positive values while MR signals from the blood become zero or negative values. Therefore, a T1W can be generated as a BB image in which the blood is depicted blackly.
On the other hand, in the case that a BB image is acquired as a T2W or a PDW, data acquisition conditions with application of multiple IR pulses, consisting of region selective IR pulses and non-region selective IR pulses, are set. Specifically, the application conditions of multiple IR pulses, including the number of pulses, application timings, and application regions of the IR pulses, are determined so that the longitudinal magnetization Mz of the static background tissues does not invert while only the longitudinal magnetization Mz of the blood flowing into an imaging slab inverts.
When a readout of MR signals is performed with a TI, with which imaging data are acquired at timing before the timing when the longitudinal magnetization Mz of the blood becomes zero due to the T1 relaxation, under such application conditions of IR pulses, the MR signals from the blood have zero or negative values. Therefore, a T2W or a PDW can be generated as a BB image in which the blood is depicted blackly.
Furthermore, in addition to acquisition of a BB image by the real part imaging accompanied by application of IR pulses, the method for generating a WB (white blood) image in which blood is depicted whitely as a high signal region has also been proposed. Specifically, the technique, which generates a WB image by inversion and maximum intensity projection (MIP) processing of brightness of a BB image, has been proposed.
In an MRA (magnetic resonance angiography) for imaging blood and blood vessels, it is desired to acquire both a BB image and a WB image with improved CNRs. However, it is difficult to obtain sufficient CNRs by the conventional method for generating a WB image using a BB image as original data. Accordingly, imaging for acquiring a WB image is separately performed by a TOF (time of flight) method or the like, in many cases.
Moreover, generating a BB image by real part imaging requires a phase correction of MR signals from background tissues. The phase correction requires image data in which not only MR signals from background tissues but MR signals from blood show positive values. For this reason, it is necessary to acquire each image for the phase correction with TI set to a long interval so that the longitudinal magnetization Mz of the blood becomes a positive value. That is, imaging for acquiring images for a phase correction is separately required in order to generate a BB image by real part imaging.
Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can acquire a necessary image, such as a BB image and a WB image, having an improved CNR with fewer number of times of imaging or less imaging time to increase efficiency of the MRI apparatus and method.